Flowers in memory of troops Click for story

The prime minister said: "These were brave men who in order to make us safer and more secure, knew the risks and had the courage to serve their country and the wider world.

"We owe them an immense debt of gratitude and our thoughts and prayers are with their families."

The troops died when a US CH-46 Sea Knight helicopter crashed in Kuwait several miles south of the Iraqi border. Defence Secretary Geoff Hoon said it was not the result of enemy action.

The Royal Navy has confirmed some of the troops killed were from their base at Stonehouse Barracks in Plymouth.

3 Commando Brigade Formed during World War II Part of the UK's Joint Rapid Reaction Force 3,500 personnel Includes three lightly armed units - 40, 42 and 45 Commando Click for 3 Commando factfile

But details of the casualties are being withheld until next of kin are informed.

Major Nick Underwood, from 59 Independent Commando Squadron, based at Chivenor in north Devon, said it was a very difficult time.

He said: "It's a tragedy and one that is a very bad start for us and for our operations, but in a war-time situation tragically these things occur and the guys out there pause, but then they will have to carry on."

Colonel Philip Wilkinson, formerly of 3 Commando Brigade, said: "It is a very sad day indeed.

"However, the procedures would have been put in place to ensure that the families of the bereaved will get maximum support in every sense, emotional and financial from the system.

"And the system will be the family of 3 Commando Brigade."

A Royal Navy spokeswoman said they were busy handling calls from anxious families following the crash.

You felt you were the only person with a child out there Carol Jones Chat with others whose family and friends are serving abroad

The navy is operating a special telephone line for relatives, 0845 7414544.

The deaths are the first confirmed casualties among US and UK troops since the war on Iraq was launched.

An investigation has begun into the crash of the helicopter which happened at about 0300 local time - 0000 GMT - south of the southern Iraqi city of Umm Qasr.

People have been leaving flowers outside 3 Commando headquarters in Stonehouse in Plymouth.

The tributes have been paid by local people who were used to seeing the Plymouth-based troops coming in and out of the barracks.

The head of the British armed forces, Admiral Sir Michael Boyce, extended his deep condolences to the families of those killed.

Conservative leader Iain Duncan Smith also expressed sympathy to the families of the dead.

Liberal Democrat leader Charles Kennedy called the deaths a "terrible tragedy".

"Everyone will salute their courage and express their deepest sympathy to their families at home," he said.

'Marine presence'

Prayers were said for the families of the dead troops when members of the congregation of St Andrew's Church in Plymouth took part in noon prayers dedicated to troops in the Gulf and their relatives.

The Rev Nick McKinnell, rector of St Andrew's, said after the service: "I think Plymouth and other cities that have a strong Royal Marine presence are particularly vulnerable because the marines are there in the thick of the action.

"We have seven weddings planned for this year where the prospective groom is now out in the Gulf. I think cities like this one are particularly sensitive."

A former US Airforce pilot told the BBC he thought the crash was probably caused by the age of the aircraft, which had been service for almost 40 years.

Troops from 3 Commando Brigade, along with US troops, have taken the strategic al-Faw peninsula in south-eastern Iraq in an air and sea assault.